1. Field of the Invention
The present invention relates to a discharge excitation type pulse laser apparatus.
2. Description of the Related Art
In a discharge excitation type pulse gas laser apparatus such as an excimer laser apparatus, pulsed discharge occurs by applying a high voltage to discharge space defined between opposing main electrodes to excite laser gas and to oscillate laser light. It is conventionally known that shock waves or acoustic waves (hereafter, to be generally referred to as “shock waves”) produced during this electric discharge cause fluctuation in the laser density in the discharge space and thus the beam profile (beam intensity distribution), beam divergence, spectral line width and so on of the laser light become instable. An example of technologies to avoid such problem is disclosed in JP H4-328889A publication. FIG. 16 is a detail drawing showing the vicinity of discharge electrodes of an excimer laser apparatus 11 disclosed in the publication. The excimer laser apparatus according to the conventional technology will now be described with reference to FIG. 16.
In FIG. 16, metallic discharge electrodes 14 and 15 are arranged in opposition to each other within a laser chamber 12 enclosing a laser gas. The upper cathode 15 is fixed to an insulating cathode base 36, and the cathode base 36 is fixed to the laser chamber 12. The lower anode 14 is mounted on an anode base 40 electrically connected to the laser chamber 12. The cathode 15 is electrically connected to the high voltage side HV of a high voltage power supply 23, while the anode 14 and laser chamber 12 are electrically connected to the ground side GND of the high voltage power supply 23. A high voltage is applied between the discharge electrodes 14 and 15 by the high voltage power supply 23 to create a pulsed main discharge in the discharge space 37, whereby pulsed laser light is generated.
During this operation, shock waves 41 are generated from the discharge space 37 by the main discharge. These shock waves 41 are reflected by components near the discharge electrodes 14 and 15, such as the anode base 40 and the cathode base 36, and return to the discharge space 37, whereby the laser gas density in the discharge space 37 is varied. As the result, the main discharge will become instable or the beam profile of laser light will be disturbed. For avoiding such problem, in the above-mentioned publication, porous ceramics 45 and 45 are fixed onto the cathode base 36 and the anode base 40, respectively. These porous ceramics 45 and 45 serve to absorb the shock waves 41 for preventing the shock waves 41 from returning to the discharge space 37.
However, the above-mentioned conventional technology has problems as mentioned below. Even if the porous ceramics 45 are disposed close to the discharge space 37, it is still difficult to absorb all the shock waves 41 generated from the discharge space 37. Thus, the shock waves 41 which have not been absorbed are propagated towards the upstream and downstream sides along the space between the porous ceramics 45 and 45. These shock waves 41 will be reflected by some sort of reflector 50 upon exiting from the space between the porous ceramics 45 and 45 and return to the discharge space 37, causing disturbance to the beam profile, the beam divergence, spectral line width and the like.
Further, in FIG. 16 for example, the shock waves 41 flying out of the discharge space 37 in a substantially horizontal direction will not be absorbed by the porous ceramics 45 and exit out from the space between the porous ceramics 45 and 45. These shock waves will also be reflected by some sort of reflector 50 and return to the discharge space 37, causing disturbance to the beam profile, the beam divergence, the spectral line width and the like.
In other words, the conventional technology has no means to prevent the shock waves 41 which have not been absorbed by the porous ceramics 45, 45 from returning to the discharge space 37 and hence poses a problem that the disturbance in the discharge space 37 cannot be prevented completely.